This invention relates to a high resolution measurement of an interval of time, and more particularly to a high resolution measuring device for a time difference suitable for use with an optical distance measuring device for calculating a distance between a body of the device and an object.
A conventional measuring device for an interval of time measures an interval of time between a starting signal and a stopping signal by counting clock signals of a reference signal of a very high frequency compared with such signals. With the measuring device for an interval of time of such conventional type, if the frequency of the reference signal is raised, then a measurement of time of a high resolution is enabled, but there exists a limitation to the frequency of the reference signal due to restrictions of electric parts and restrictions on circuit construction. Thus, the interval of time between the starting signal and the stopping signal is measured by a plurality of times using a reference signal which is not synchronized with those signals. In particular, if a plurality of starting signals and stopping signals are examined in a fixed condition as shown in FIG. 10, clock signals for each measurement are displaced from each other by an amount corresponding to an out of synchronism amount. Thus, if N times of measurement are performed while counting clock signals CL1 to CL5 between a starting signal and a stopping signal by means of a counter, then the total number .SIGMA.CL is equal to a value of a count when a measurement is performed once with clocks of a frequency of N times the frequency of the clock signals CL1 to CL5. Accordingly, this signifies a rise of resolution by N times.
However, a time interval measuring device of the conventional type is required to repeat a measurement by N times in order to raise its resolution by N times. In particular, this signifies that N times a repeat time of a measurement signal are required for a time for measurement, and if it is intended to measure, for example, an interval of time between a starting signal and a stopping signal having a frequency of 1.5 KHz with a resolution of 6,000 times that of the clock signal, then there is a problem that a period of time of ##EQU1## is required. Further, the timing of a stopping signal is sometimes varied by a noise produced within the measuring device or an external factor such as measuring circumstances, and against such variations, the number of measurements must be increased to perform an averaging processing. With a conventional measuring device, however, there is a problem that an averaging effect cannot be raised sufficiently because of a restriction in time for measurement. Particularly where a conventional time interval measuring device is employed for optical measurement, a stopping signal is produced in response to light reflected from an object, but such stopping signal is sometimes varied by a noise within the device or by a change in refractive index of air. By the way, if it is assumed that the stopping signal is varied by .DELTA.t, the clock signals CL4 and CL5 contribute to such an averaging effect as described above, but the other clock signals CL1 to CL3 do not contribute to the averaging effect. Accordingly, where a stopping signal is being varied, clock signals are divided into those clock signals which contribute to an averaging effect and the other clock signals which do not contribute to an averaging effect, and the clock signals which do not contribute to an averaging effect do not cause any change in .SIGMA.CL. Accordingly, there is a problem that the averaging effect is generally low. Particularly where a pulse laser diode is employed as a light source of an optical distance measuring device, it has a duty ratio of 0.01% or so. Accordingly, there is a problem that an adequate averaging effect cannot be anticipated.